This invention relates to NMR systems. More specifically, the invention relates to a system for precisely aligning a patient on a table prior to performing an NMR study and for providing reference data useful in acquisition of NMR data and in image manipulation.
An NMR system typically is made up of a magnet, a table for supporting a patient, and digital computer apparatus for controlling system operation, data acquisition, and subsequent image reconstruction, for example. The magnet, which may be of resistive or superconductive construction, is generally configured as a cylinder having a longitudinal bore approximately 90 cm. in diameter. A shim coil assembly located within the bore is used to correct magnetic field inhomogeneities in the main field. A gradient coil system with a typical diameter of about 65 cm. is situated within the magnet bore interiorly of the shim coil assembly. Gradient coils are needed in NMR imaging systems to provide substantially linear magnetic-field gradients used to encode spatial information into NMR signals. A radio-frequency (RF) body coil used for exciting the nuclear magnetic resonance phenomenon and for receiving NMR signals further limits useful bore diameter to approximately 55 cm. This is sufficient for accommodating most patients, but severely limits the ability to move the patient both transversely and vertically within the bore.
To obtain optimal NMR image quality, the patient region of interest must be positioned within a centered spherical volume of approximately 40 cm. in diameter centered about a system isocenter where the magnetic field produced by the magnet is most homogeneous. This is accomplished by using a patient support device capable of bidirectional longitudinal travel within the magnet bore but which does not travel in the transverse or vertical directions. The use of such a device has its design advantages since transverse travel is severely restricted in any case by the RF body coil diameter. Vertical travel is not only limited, but is difficult to implement because of the extreme longitudinal travel range (about 300 cm.) and the requirement that the patient support system be capable of supporting a 300 pound load. A problem which arises due to the lack of vertical travel capability (i.e., without the ability to center the patient volume of interest with the isocenter) is that the image reconstruction process lacks the necessary reference information to an image center for the purpose of displaying images centered in the field of view or for reconstructing magnified and unmagnified images which are offset from the image center. The manner in which offset images are reconstructed is claimed and disclosed in copending, commonly assigned U.S. patent application Ser. No. 555,097 now U.S. Pat. No. 4,593,247, filed by Gary H. Glover, which is incorporated herein be reference.
It is therefore an object of the present invention to provide a patient alignment system for precisely positioning a patient longitudinally in a region of interest within the magnetic field volume having optimum homogeneity.
It is another object of the invention to provide a patient alignment system capable of providing reference information for displaying centered images even when the region of interest is not centered with the isocenter.
It is a further object of the invention to provide a patient alignment system for referencing an anatomical reference point which is recorded and which may then be used as a reference for determining the dimension and position of the region to be studied.